A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
In addition to generating a desired circuit pattern, the patterning device may be used to generate metrology targets on the target portion of the substrate. A metrology target may be a feature or set of features that is applied to the substrate for the purpose of metrology, for example to facilitate determination of measures of overlay or focus quality.
In order to ensure that pattern features applied to a substrate are applied as intended (e.g. to ensure that critical dimension limits, requirements, or uniformities are met and/or to ensure that overlay requirements are met), it may be desirable to at least partially correct for optical aberrations in the lithographic apparatus. The determination and control of optical aberrations may be important for improving lithographic performance. As overlay performance is becoming more demanding, lens aberrations may be becoming a limiting factor to performance in some circumstances.
Aberrations may arise due to heating of one or more elements of a projection system of the lithographic apparatus due to the transmission or reflection of at least a portion of a radiation beam, and this heating may cause distortion or the like of those one or more elements. Alternatively and/or additionally, aberrations may arise for one or more other reasons, for example optical surfaces not performing in accordance with theory.
Aberrations may not only affect the overlay of device features directly by pattern shift, but may also affect the accuracy of metrology targets due to mismatched sensitivities with the device features. A metrology target may be used to measure overlay on the assumption that the measured overlay of the metrology target is representative of the overlay of a device in the neighbourhood of that metrology target. If the metrology target is more or less sensitive to a particular aberration than the device itself, then the overlay measured on the metrology target may differ from the overlay present in the device. A correction applied by an APC (advanced or automated process control) in response to a measured metrology target overlay may reduce the metrology target overlay, but add an overlay error to the device. This may force customers to include offsets in their process control loop to correct for the difference so that the device overlay is properly corrected. Such offsets may correct the mismatch between metrology overlay and device overlay.
The impact of different types and magnitudes of aberrations is application specific. How a specific application (e.g. an application of a pattern to a substrate) responds to a certain aberration may be defined as aberration sensitivity. Aberration sensitivity may depend on one or more of a number of factors, for example an illumination mode used by the lithographic apparatus, one or more properties of a pattern feature to be applied to a substrate, one or more features of the substrate itself (for example, the composition or like of resist), the quality or configuration of the patterning device, and a dose of radiation provided in any given exposure.
Since aberration sensitivity is an important factor that needs to be taken into account when applying patterns to a substrate, there have been attempts to determine aberration sensitivity of such patterns. One approach to determining aberration sensitivity is to construct a model or simulation which allows sensitivity to be determined in a theoretical model.
Some experimental methods are also known. In an example of a known experimental method, pattern features are applied to each of a number of target portions of a wafer (which may be different fields of a wafer) in turn. Amongst possible methods, lens heating feed-forward is used to induce an aberration and to gradually increase the magnitude of the aberration as the exposure progresses, such that target portions of the substrate that are exposed first are exposed with low magnitudes of the aberration, and target portions of the substrate that are exposed subsequently are exposed with successively higher magnitudes of the aberration. Once the pattern features have been exposed and appropriately processed, one or more properties of the pattern features of the target portions are measured, for example a sharpness of a pattern feature, a dimension of a pattern feature, or a shape of a pattern feature. The measurements from different target portions (corresponding to different magnitudes of the aberration) are used to determine a sensitivity of the property or properties of the pattern features to changes in the magnitude of the aberration.
In order to get a good signal to noise ratio, it may be required to measure a large number of data points on the wafer and correlate the induced aberrations with a measured property or properties, for example overlay. When exposing a wafer in a standard fashion with lens heating feedforward used to induce aberrations, it may only be possible to get a single sensitivity (that is, the sensitivity of a single property to a single aberration component, for example a single Zernike coefficient) measured from that wafer in order to get a good signal to noise ratio. Multiple sensitivities may be determined by exposing successive wafers, each with a different aberration.
The time needed to get an overlay measurement may be large since the number of pattern features used for the measurement may be high. In lens-heating feedforward, a Zernike offset is gradually increased. In order to determine sensitivities, multiple points may be needed so that a linear relation between overlay and aberration may be determined. Having more points may make the relation more accurate.
Measuring multiple sensitivities may in some circumstances require a large wafer cost and fab usage for the customer.